Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M16: Focus Session: Computational Studies of Thermoelectric Materials |
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Sponsoring Units: DCOMP Chair: Emmanouil Kioupakis, University of Michigan Room: 401 |
Wednesday, March 5, 2014 11:15AM - 11:51AM |
M16.00001: Finding new thermoelectrics: Parabolic bands are (probably) not enough Invited Speaker: David Joseph Singh Thermoelectric performance as characterized by the figure of merit ZT is a counterindicated property of matter. While the electronic structure of common semiconductors is well understood in terms of band models, most commonly the parabolic band model, this type of electronic structure is not the best for finding high thermoelectric performance. Instead high ZT thermoelectrics often have unusual band structure features. Here I discuss some of those features, and their essential aspects in relation to thermoelectric performance and outline strategies for finding more high ZT materials based on them. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M16.00002: Quantum Resonance Effects to Thermoelectric Property of Organometallic Molecular Materials Hisao Nakamura Superior long-range electric transport has been observed in several organometallic wires and films. Here, we propose use of organometallic molecules for thermoelectric materials by focusing on the overlapping resonance effect, which enables long-range coherent tunneling and enhancement of Seebeck coefficient. We examine the possibility of high thermoelectric figure of merit (\textit{ZT}) by controlling the quantum resonance based on first principles transport calculations of electron and phonon. [1] We found distinct length and temperature dependences of \textit{ZT} from those of inorganic bulk materials or organic molecules. We will present an alternative approach to obtain high \textit{ZT} by using organometallic molecular materials. \\[4pt] [1] H. Nakamura, T. Ohto, T. Ishida, and Y. Asai, \textit{J. Am. Chem. Soc.} DOI: 10.1021/ja407662m [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M16.00003: First Principles explanation of the positive Seebeck coefficient of lithium Matthieu Verstraete, Bin Xu Lithium is one of the simplest metals, with negative charge carriers and a nearly free electron dispersion. Experimentally, however, Li is one of a handful of elements (Cu, Ag, Au) where the sign of the Seebeck coefficient ($S$) is not that of the carrier. We calculate $S$ fully from first-principles, within P.B. Allen's formulation of Boltzmann theory. The constant relaxation time approximation fails and gives a sign for $S$ necessarily identical to the carriers. Our calculated $S$ are in excellent agreement with experimental data. In comparison with Na, we demonstrate that, within the simplest non-trivial model for the energy dependency of the electron lifetimes, the rapidly increasing density of states (DOS) is related to the sign of $S$. The exceptional energy dependence of the DOS is beyond the free-electron model, and distorted by the Brillouin Zone edge, a stronger effect in Li than other Alikis. The electron lifetime dependency on energy is central, but details of the electron-phonon interaction are less important, contrary to what has been believed for several decades. The mechanism exposed here may open the door to new ``ambipolar'' thermoelectric materials, with a tunable sign for the thermopower even if either n- or p-type doping is impossible. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M16.00004: Thermoelectric transport through single molecule junctions Marius Ernst Buerkle, Thomas Hellmuth, Fabian Pauly, Yoshihiro Asai The charge and heat transport properties of single multilayered cyclophane stacks connected to gold electrodes are calculated in the framework of ab initio electronic structure calculations combined with non-equilibrium greens function techniques. The heat transport include both the electronic and phononic contribution, which allows a fully ab initio determination of the thermoelectric transport properties. We investigate the influence of the molecular length and of the molecule-electrode binding motif on the electron and phonon transport characteristics. We find that the power factor is limited due to the fact that the Wiedeman-Franz law remains approximate valid due to the strong off-resonant electron transport. On the other hand the large phonon mismatch between the molecule and the gold electrodes leads to a suppression of the phonon thermal conductance. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M16.00005: First-principles study of the thermoelectric properties of Cu2S Keenan Zhuo, Cheng-Rong Hsing, Ching-Ming Wei, Mei-Yin Chou The mineral chalcocite, or copper sulphide (Cu$_2$S), is of interest as a thermoelectric material due to its abundance and non-toxic nature. Yet, the study of Cu$_2$S is complicated by the disordered phases (hexagonal and face-centered cubic) that it exists in at high temperatures. Here, we discuss our random structure search leading to the most stable structures. Based on these results, we report the thermoelectric properties of hole doped Cu$_2$S using first-principles calculations and Boltzmann transport theory. We show that a high Seebeck coefficient of over 200 $\mu$V/K is achievable with hole doping levels up to $10^{20}$ cm$^{3}$ above 500 K. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M16.00006: Atomic simulations of nonlinear lattice dynamics in PbTe Yue Chen, Chris Marianetti PbTe is of great interest as a thermoelectric material and for displaying signs of strong phonon interactions. Inelastic neutron scattering experiments reveal a signature of strong anharmonicity as evidenced in anomalous temperature dependence of the phonon spectra. Here we perform molecular dynamic simulations using a 4th-order interatomic potential deduced from first-principles calculations. The temperature dependent phonon spectra are successfully reproduced from first-principles for the first time. The emergence of a new mode at the zone center is unambiguously shown, as observed in experiment. Furthermore, we confirm that there is not a local spontaneously broken symmetry, clarifying recent controversy among experimental results. Phonon self-energies at different temperatures are computed to show the origin of the phonon anomalies. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:27PM |
M16.00007: Computational design for low-temperature thermoelectric materials Invited Speaker: Mona Zebarjadi Thermoelectric materials are usually doped with external impurity atoms which provide the required level of carrier concentration (electrons/holes) for a good electronic performance. These impurity atoms scatter the conduction carriers and limit their mobility. Such limitation can be improved by introducing new doping schemes. For instance, impurity atoms can be substituted by metallic/ semi-metallic nanoparticles, or heavily doped semiconducting grains/nanowires can be embedded inside a host matrix in order to create a three dimensional modulation doping structure. We have recently demonstrated three dimensional modulation doping scheme in nanostructured SiGe materials and observed about 40{\%} enhancement in the carrier mobility compared to uniform doping. The enhancement could be much larger if a complete separation of carriers and ions is achieved e.g. by addition of a spacer layer It is possible to shield the nanoparticles with a coating layer to minimize the conduction carrier scattering and reduce the scattering cross section by 4 orders of magnitudes below the physical cross section to cloak the nanoclusters and to design invisible dopants. Extension of such a design to realistic materials can increase the carrier mobility by orders of magnitude especially at low temperatures, and can potentially increase the thermoelectric performance by two orders of magnitude. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M16.00008: An ab initio study of the effect of host-guest interaction on thermal transport in Ba$_{8}$Ga$_{16}$Ge$_{30}$ Terumasa Tadano, Yoshihiro Gohda, Shinji Tsuneyuki Inorganic clathrate compounds are promising candidates for the next-generation thermoelectric devices because of their low lattice thermal-conductivities. In these materials, rattling vibrations of guest ions inside host cages are considered to play a significant role in reducing the lattice thermal-conductivity. In order to elucidate the microscopic mechanism of the reduction more clearly, we have performed first-principles analyses on a type-I clathrate Ba$_{8}$Ga$_{16}$Ge$_{30}$. Firstly, we calculated harmonic and anharmonic force constants of the material using the direct-method. Then, phonon scattering probabilities are evaluated from the imaginary part of the phonon self-energy. Our analysis shows that host-guest interactions increase the scattering probability of acoustic modes by one order of magnitude, and also cause a 10-fold reduction in the lattice thermal-conductivity. In addition, we observe that phonon mean-free-paths are far larger than the separation of Ba atoms, indicating that Ba atoms cannot be considered as individual scattering centers. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M16.00009: Spherical Harmonic Expansion Method for Coupled Electron-Phonon Boltzmann Transport Marco Santia, John Albrecht Thermoelectric transport modeling often relies on independent Boltzmann transport equations (BTEs) for electrons and phonons which work best near equilibrium (linearized) and steady-state. Device design relies heavily on this baseline approximation. Monte Carlo methods can allow for complex physical interactions (e.g., anharmonicity) but their stochastic nature has practical limits. Distribution functions with wide disparities in population (e.g., ratios $> 10^8$ between majority and minority carriers.\footnote{The SHE method has treated majority/minority carriers in bipolar transistors, S.-M. Hong, et al, IEEE Trans. Electr. Dev. \textbf{57}, 2390 (2010).}) are a computational challenge. We present a coupled BTE solver based on a k-space spherical harmonic expansion (SHE) of the distribution functions and eigenstates of electrons and phonons. The method is deterministic and allows for detailed treatments of scattering processes, yet ameliorates the issues with population disparity within phase space. We set the formalism and examine the accuracy of the SHE for phonon band structures, calculate scattering rates determined within that representation, and compare our preliminary results for distribution statistics in control examples such as thermal conductivity and drift velocity. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M16.00010: Temperature dependent anharmonic lattice dynamics Olle Hellman, Sergei Simak, Igor Abrikosov We have developed a thorough and accurate method of determining anharmonic properties in solids, the temperature dependent effective potential technique (TDEP). It is based on ab initio molecular dynamics followed by a mapping onto a model Hamiltonian that describes the lattice dynamics. The effective Hamiltonian contains implicit temperature dependence, electron phonon coupling and renormalized anharmonicity to arbitrary order, making it suitable for strongly anharmonic systems. We show excellent results for a host of thermoelectric materials (PbTe, SnTe, Bi$_2$Te$_3$, FeSi, ScN), reproducing temperature dependent phonon spectra, thermal conductivity, and phonon self energies. [Preview Abstract] |
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